Just to add why: the space after \tI gets ignored while reading the input (it follows a command name) and the error line 0.0sub shows it. However, the common trick of putting {} after the command can't work here, while \space does the right thing.
–
egregSep 29 '12 at 17:47

Now let's discuss your problem. Control words will eat spaces following them. So x \tI sub becomes x 0.0sub that is no meaning. To preserve the spaces, you need to use \space as shown in the code above.

Another solution:

Make sure you put at least one space before the closing } in your macro definition. See the code below.

\documentclass[pstricks,border=12pt]{standalone}
\usepackage{pst-plot}
\begin{document}
\begin{pspicture}[showgrid](-1.5,-1)(1.5,1.5)
\def\tI{0.0 }% can you see there is one space before the closing }?
\psplot{0.0}{1.0}{x \tI sub}
\end{pspicture}
\end{document}

This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post.
–
Tom BombadilSep 29 '12 at 19:21

it could be good idea that you read the question!
–
HerbertSep 29 '12 at 19:24

1

It's the same what ガベージコレクタ did and also commented in his code. So your answer is nothing additional and should, in my humble opinion be a comment to ガベージコレクタ's answer.
–
Tom BombadilSep 29 '12 at 19:26